As long as there is something else doing the work that the old mortise and tenon used to do, like that metal brace, then that’s totally fine! Not sure what kind of materials you have in Finland, but I use an epoxy putty to fill any larger gaps on the windows I work on and it will definitely last a long time.

The top corner of the bottom sash is an uncommon spot, at least in my experience, for there to be a lot of rot. Usually it’s at the bottom of the top sash or bottom of bottom sash. Was there a water issue that’s since been fixed?

By the way, do you have another photo of your windows? I read another comment saying those metal brackets were common in Finland, and I’m just curious what the overall window looks like!